Here's a few of rules of thumb I've developed through experimentation over the years that work for me....
1. Look at the point where the trailer hitches to the bike, and the points at which the trailer's wheels touch the ground. They form a triangle, right? OK, you want to keep the sides of the triangle reasonably close to being the same length. If the side of the triangle that lies along the trailer's axle line is the base then I would keep the height of the triangle (i.e. the distance from the trailer axle line to the hitch) to a maximum of about 1.5 to 2 times the base.
2. You want the center of gravity of the trailer (especially when loaded) to fall pretty much on the axle line of the trailer wheels so that there's very little upwards or downwards force at the hitch. that means that you want the wheels pretty close to the middle of the trailer, not all the way back down the end.
3. You want to make the trailer as low and as wide as reasonably possible so as to keep the center of gravity (when loaded) as low as possible, the lower it is the more laterally stable it is. I try to stay below 3:1 width to height ratio.
The one big variable here is just how much stability do you want? I often take sweeping bends at up to 20mph with 50lb on the trailer on less than perfect surfaces and I can hardly tell its there. I occasionally even take a short cut home from the grocery store down a stretch of singletrack. That requires a nice stable design and a good hitch with no slop. You may be perfectly happy with something less than that.